The present invention generally relates to base stations, a radio controller for controlling the base stations and a mobile communication system including them, and, more particularly, to a mobile communication system which can control downlink transmission power from base stations to mobile stations.
In a mobile communication system such as the CDMA (Code Division Multiple Access) system in which so called soft handoff is available, mobile stations can simultaneously communicate with nearby plural base stations in a soft handoff.
Generally, in mobile communication systems, radio links established between base stations and mobile stations are always varying due to fading. In order to suppress such fading effect, a mobile station in a soft handoff sends an instruction (such as a downlink transmission power control instruction, for example, a TPC bit) to plural base stations communicating with the mobile station itself to request the base stations to control their downlink (base station to mobile station) transmission powers. The base station, in response to the downlink transmission power control instruction, increases or decreases its downlink transmission power and performs power balance control for the purpose of balancing the downlink transmission power.
On the other hand, a base station sends an uplink (mobile station to base station) transmission power control instruction to mobile stations communicating with the base station itself. The mobile station, in response to the uplink transmission power instruction, increases or decreases its uplink transmission power.
In closed loop control for transmission power control, a mobile station in a soft handoff transmits downlink power control instructions with the same transmission power to plural base stations which establish links between the plurality of base stations and the mobile station.
Therefore, if the transmission loss of the uplink from the mobile station to a base station is small, the base station can correctly receive the downlink power control instruction. However, if the transmission loss of the uplink is large, the base station may fail to receive the downlink power control instruction. As a result, the transmission power values of the downlinks established between the plural base stations and the mobile station may become uneven.
Consideration is given to problems that might occur when the transmission power values of the downlinks established between the plural base stations and the mobile station become uneven. For example, assume that a base station fails to receive a downlink power control instruction from a mobile station due to a large uplink transmission loss, and then the downlink power of the non-receiving base station becomes lower than the downlink power of another base station experiencing a small uplink transmission loss. In this case, mobile stations tend to fail to receive an uplink power control instruction from a base station experiencing a large uplink transmission loss. However, because uplink transmission power values are mainly controlled by an uplink power control instruction from a base station experiencing a small transmission loss, the problem is not so serious.
On the other hand, assume that a base station fails to receive a downlink power control instruction from a mobile station due to a large uplink transmission loss, and then the downlink power of the non-receiving base station becomes higher than the downlink power of another base station experiencing a small uplink transmission loss. In this case, the downlink power of the non-receiving base station may become larger than necessary, resulting in interference to other mobile stations within the cell being serviced by the non-receiving base station and therefore low downlink capacity.
In order to solve such a problem, Japanese Patent Laid-Open Publication No. 11-340910 discloses a technique, in which whenever a base station receives a downlink power control instruction, the base station changes its downlink power responding to the power control instruction and controls the downlink transmission power value so as to approach a predetermined value. In the '910 Publication, the maximum transmission power, the minimum transmission power, the mean value between the decibel values of the maximum and minimum transmission powers in the base station, and the statistical value of the base station transmission power are utilized as a reference value.
Japanese Patent Laid-Open Publication No. 2002-232353 discloses another technique, in which base stations are separated into two groups, one group having high downlink communications quality and another group having low downlink communications quality. Power balance control is continued fort the high quality group. Downlink transmission power values of base stations belonging to the low quality group are decreased to a predetermined level, in order to lower the interference against the high quality downlinks and increase the downlink capacity.
However, the '910 Publication does not consider a situation where downlink communications quality of each base station is different from the others. Therefore, for example, if a base station increases its downlink transmission power value due to a degraded downlink communications quality level, then once the increased downlink transmission power value is determined as a reference value, other base stations' downlink power values become higher than necessary, resulting in wastefulness.
The '353 Publication does not consider how to determine a reference value for transmission power values in power balance control.